1. Field of the Invention
This invention relates to an improvement in a gaseous phase synthesis process of diamond, synthesis apparatus for this process and synthetic diamond obtained by this process and more specifically, it is concerned with a high quality synthetic diamond with a high bonding strength between the synthetic diamond and a substrate surface, or between synthetic diamond films and with an excellent heat conductivity.
2. Description of the Prior Art
As a method for the production of diamond by a gaseous phase synthesis, a number of methods are known such as microwave plasma CVD methods, high frequency plasma CVD methods, DC plasma jet methods, etc., but as a method on commercial scale, thermal filament CVD methods excellent in mass-productivity and feasible with a low cost installation have commonly been employed.
The thermal filament CVD method is a synthetic method for obtaining a diamond film, comprising subjecting mixed gases of a hydrocarbon and hydrogen to thermal cracking by thermoelectron emission of a heating body heated at 1000.degree. C. or higher and then depositing it on a substrate surface heated at 500 to 1300.degree. C.
In a filament CVD method of the prior art, diamond has been synthesized by the following steps:
(1) placing a heating body and substrate with an interval therebetween suitable for the synthesis of diamond, PA1 (2) introducing mixed gases of a hydrocarbon and hydrogen and gradually heating the heating body and PA1 (3) when the temperature of the heating body reaches a temperature suitable for the synthesis of diamond, maintaining the heating body at a constant temperature and synthesizing diamond on the substrate.
The filament CVD method has a problem of metallic contamination of the synthetic diamond because of using, in general, a high melting point metal such as W, Ta, Re, etc. as a heating body. Since the heating body is covered with carbon atoms as a raw material at a certain temperature and the surface of the heating element is protected by the so-called carburization treatment, the metallic contamination of the synthetic diamond is decreased, but the metallic contamination in the step of heating the heating body until the surface of the heating body is completely covered by this carburization treatment has become an important problem.
That is, the substrate surface or previously synthesized diamond surface is coated with the metal and stripping of the diamond film during or after the synthesis sometimes takes place due to low bonding strength of the metallic coating. Furthermore, the heat conductivity is lowered to deteriorate the property as a heat radiation substrate because of the presence of the metallic coating part in the synthetic diamond film.